Method of growing crystals



M'arh 27, 1951 o. KORNEI METHOD OF GROWING CRYSTALS Filed March 21, 1945 A A m n ro m 6 n w I m ww L\ M A 3 f n fi m 2 Z FIG. I

FIG. 3

INVENTOR. OTTO KORNEI RNEY ATT

Patented Mar. 27, 1951 METHOD OF GROWING CRYSTALS Otto Kornei, Cleveland Heights, Ohio, assignor to The Brush Development Company, Cleveland, Ohio, a corporation of Ohio Application March 21, 1945, Serial No. 584,009

My invention pertains to methods of growing crystals.

Two basic methods have been utilized for causing a crystal to grow from a solution. One is to gradually reduce the solubility of the solute, such as by reducing or, in some cases, by increasing the temperature of the solution, and the second is to gradually remove solvent from the solution, such as by evaporation. In either case the solution becomes more and more concentrated, then saturated, and finally supersaturated. When a certain degree of supersaturation is obtained all of the solute cannot be maintained in solution and some of it crystallizes out, either spontaneously as a large number of small crystals or slowly onto a small number of seed pieces of the crystalline material thereby leading to the growth of a small number of large crystals.

The first method is not universally operable as .some substances have a solubility which changes only very little with temperature. Thus decreasing or increasing the temperature of a solution will have very little influence on crystallization therefrom. Also, this method cannot be made continuous due to the necessity of cyclically heating and cooling the solution.

The evaporation method, while it can be made practically continuous and Works for most crystallline materials, has certain practical difficulties. tact with a gas (e. g., air) whose relative humidity is low. To maintain effective control over the humidity of the gas requires considerable equipment; the necessarily large 'free surface of the liquid in contact with the dry gas may also lead to the spontaneous formation of small crystals on the surface due to dust or dirt particles which are likely to accumulate and act as nuclei in this crystallization. If it is the object of the process to grow large crystals these small surface crystals are highly undesirable as they settle down and interfere with the growth of the planted seed pieces of crystalline material.

My invention provides a method, and the means for practicing the method, which combine the best features of the evaporation and temperature change methods of growing crystals.

The method is, consequently, primarily advantageous if crystals are to be grown at substantially constant temperature or if the substance to be crystallized shows only little change of its solubility with temperature. The growing of large crystals of substances with a very low absolute solubility is also facilitated by the method because of the Simplicity and speed by which The surface of the solution must be in con- 3 Claims. (01. 23-301) large quantities of solvent can be removed from the solution.

Another object of my invention is to provide a method of growing crystals, particularly large single crystals, which does not require a change in the temperature of the solution.

A further object of my invention is to substantially continuously grow crystal from a solution wh ch is maintained at substantially constant temperature.

Other objects and a fuller understanding of my invention may be had by referring to the following description and claims, read in conjunction with the drawing wherein:

Figure 1 schematically illustrates means for growing crystals utilizing osmotic removal of solvent fromthe crystal solution, and

Figure 3 illustrates a modification of the device shown in Figure 1.

Figure 2 illustrates a modified form of the invention wherein a substantially continuous process is possible.

Figure 1 illustrates a simple device for growing crystals utilizing osmotic removal of solvent from a solution. Reference character 5 indicates a container which is divided into t o compartments 6 and 1 by a semi-permeable wall or membrane 8.

It should be remembered that a semi-permeable Wall or membrane, generally sneaking, permits the passage of the solvent of a solution but not of the solute. The perfection of this separation depends upon the nature of the membrane; the speed of the process depends also upon the membrane and in addition upon the difierence between the osmotic pressures existing in the two solutions in contact with the opposite faces of the membrane. The osmotic pressure of a solution, in turn, is determined by its molar concentration and by the temperature. For dilute solutions of non-dissociating materials (non-electrolytes) the osmotic pressure is directly proportional, according to Vant Hoiis law, to the molar concentration and to the absolute temperature. Accordingly, the osmotic pressure of such a solution at room temperature, turns out to be approximately 25 atmospheres for a concentration of 1 gram-mole per liter solution.

For high concentrations and/or for solutions containing electrolytes the interdependence between concentration and osmotic pressure becomes more complicated and has to be established empirically in most cases.

Another factor by which the speed of transfer of liquid through the semi-permeable membrane can be increased is the application of a steady electric potential diiference across the two cells which are separated by the membrane. The ensuing action is called electro-osmosis and it may be advantageously used in any of the modificat ons of the described crystal growing method. The basic set-up for practicing this method is shown in Figure 3, referred to later.

The compartment 6 in Figure 1 contains a first solution 9 and compartment 1 contains a second solution I3. The second solution may comprise some crystalline material dissolved, preferably, in the same kind of solvent which is in the first solution. In order that osmotic transfer of solvent from compartment 6 to compartment I be effected it is required that the second solution I have a higher osmotic pressure than the first solution 9. Pure solvent from solution 9 will then gradually pass through the semi-permeable wall 8 into the solution I9, thereby reducing the concentration and osmotic pressure of solution ID. If, however, the osmotic pressure of solution I9 is, in the beginning, considerably higher than the osmotic pressure of solution 9, then the process will contine sufficiently long to cause solution 9 to become super-saturated and some of its salt will crystallize out. Seed pieces of the crystalline material to be grown may be planted in the solution 9 to facilitate the growth of a small number of large crystals.

The osmotic transfer of solvent will raise the level of the solution in compartment I and will decrease it in compartment 6. The resulting hydrostatic pressure difference between the liquids in the two compartments will, consequently, reduce the efiectiveness of the osmotic action, and the action will continue only until the difference in osmotic pressures is counterbalanced by the diiference in hydrostatic pressures.

Figure 2 illustrates sc emati ally a device which does not allow the establishment of an a preciable hydrostatic pressure difference between the two compartments, and which maintains saturation in both of the solu ions all the time. Ac-

cordingly, the process will continue substantially indefinitely and large crystals can be grown.

A container II holds a large supply of solution I2 of the crystalline material which is to be grown into crystals. Within the container I I there is suspended a second container I3 by means of supports I5 the ends of which are fastened to the sides of the container II. The bottom of the second container I3 is comprised of a semi-permeable wall or membrane 3 and is positioned below the surface of the liquid I2. Seed pieces 2! of the crystalline material may be planted in the solution I2 to facilitate the growing of a predetermined number of large crystals. The saturated solution I6 must have a higher osmotic pressure than the saturated solution I2. Within the container I 3 there is placed a surplus of the solute I! of the saturated second solution I6 so that the solvent which passes through the membrane 8 will not dilute the second solution It, which would result in a reduction of its osmotic pressure. In this continuous process it is essential that the two solutions I2 and I6 have the same solvent; therefore, the solute I? must be soluble in the solvent in which the crystalline material is dissolved.

The container II cover I8. tight packing 29 in the cover I8 into the container I3, terminating at the normal level of solution I6. A valve 25 is provided in the p pe is closed by an air-tight A pipe I9 extends through the air' I9 and the pipe leads into a sealed evaporating jar or chamber 22. Another pipe 23 extends into the evaporating jar 22 and its end 24 is positioned above the level of the solution I3 within the jar. A heater 26 is provided for vaporizing the liquid within the jar 22; the remaining residue II which results upon the vaporization of the solvent from solution I6 gathers at the bottom of the jar. The pipe 23 passes through a cooling unit or condenser 27 which has inlet and outlet openings 28 and 29 whereby cooling water may be passed around the pipe 23 to cause the vapor within the pipe to liquefy. The outlet of the condenser is connected to the inlet of a pump 39 and the pump outlet is connected to a pipe 3| which extends into the sealed jar 32.

The jar 32 contains a supply 33 of the material which is to be crystallized and a pipe 34 extends from the liquid level in the jar 32 through an airtight packing 35 into container II, terminating above the level of solution I2. Thus a circuit is established from container I3 through container 22, condenser 2?, pump 39, container 32 back to container II, and this circuit may be considered to be closed by the semi-permeable membrane 8 which permits solvent to pass from the container II to the container I3.

When the system is in operation the solvent of the saturated solution I 2 in container I I passes by osmotic action through the membrane 8 leaving its solute behind to cause a further increase in the concentration of the solution I2 and consequent growth of the crystal seeds 2|. The saturated solution I5, containing the same kind of solvent as solution I2, does not become diluted by the incoming solvent because of the excess of solute I? at the bottom of conainer I3. This excess dissolves gradually as more solvent enters container I3, thus re-establishing continuously saturation in solution I6. The level in container I3, at the same time, is maintained constant because the pipe I9, due to the suction established by pump 39, keeps the solution IS in the container i3 at the normal level by removing accumulated excess solution IE to the jar 22. The heater 2% vaporizes the solvent of solution I3, the vapor being drawn off through pipe 23 to the condenser 2? where it is returned to the liquid state, and the solute I'I remains in the jar 22. In order to facilitate vaporization of the solution It in jar 22 the valve 25 may be partially closed resulting in a pressure drop across it. A reduced pressure is thus obtained in the jar 22 resulting in a lowering of the boiling point temperature of the solution I6, therein. This prevents decomposition of compound II which might otherwise take place.

The solvent which comes out of the condenser 21 is used to again form a saturated solution of the substance to be crystallized by dissolving a quantity of substance 33' in jar 32, this substance being identical with the solute in solution I2. The solid material and solvent within the jar 32 may be stirred to facilitate the formation of fresh saturated solution. When equilibrium of the liquid flow is reached the pipe 34 extends to the surface of solution I2 in jar 32. Pump 39 causes the accumulating surplus of solution I2 to flow into the solution I2 in container I I, thereby continuously supplementing the solution I2 With exactly the same amount of liquid which is being lost due to crystallization of solute and osmotic transfer of solvent through the semipermeable wall 8. In order not to disturb the equilibrium of solution I2 in container II, the

fresh saturated solution coming from container 32mustbe-broughtto substantially the temperaof solution I2;before being addedto it." This may :be, done by controlling the temperature of Waterprythe like within the jacket 36 surround ing' the pipe 34. 'Hot-and cold water may be supplied to the jacket '36, and-the flow thereof may be thermostatically controlled to regulate the temperature of the solution in pipe 34 in accordance with the temperature of the solution I2.

By this process the liquid-levels in the two containers ii and I3 are kept equal to prevent the osmotic transfer of solvent from slowing down which would occur if a difference in hydrostatic pressures in the twocontainers were allowed to build up. Also, the saturation of solution 15 in container i3 is maintained to keep the osmotic pressure difference between the two solutions constant.

1 When the crystals have grown to a sufficient sizethe cover 18 of the container -,II is removed, the crystals are taken out, the materials l! and 33 are replenished, new seeds are planted and the cover is replaced. The apparatus is then ready for another run. It is'to be noted that the material .Ilis never used up. It is merely transferred. d ring the process, from container l3 into container 22 from which it can be returned into container l3 when required.

It is desirable that the seeds 2| be placed beneath the membrane 8 as it has been found that the supersaturated solution established at the underneath side of the membrane is sufficiently heavy with respect to the saturated solution that it moves downwardly without depositing crystalline material until it comes in contact with a seed about which growth will occur. The seeds 2|, mounted on pedestals above the bottom of the container H, act as nuclei and the crystalline material is thus caused to grow on these seeds.

In order to promote more rapid and uniform growth of the crystalline material the liquid contents of containers l l and I3 may be agitated,

for instance by stirring or rocking. Reference may be had to the Kjellgren Patent No. Re. 19,697 for a detailed description of rocking the liquid.

The membrane or semi-permeable wall 8 may consist of any one of a number of materials. One way of producing such a membrane or wall is to hem cally d posit a colloidal metal-ferrocyanide within the. pores of a plateof porous ceramic. The metal may be copper, nickel or the like, and the ceramic may be clay, unglazed porcelain, sintered glass, or any other suitable porous material. active semi-permeable member in the form of many tiny membranes within the pores of the porous material which acts as a skeleton to support the membranes. Other types of semipermeable walls may be formed of self-supporting films or skins of natural or of artificial origin such as animal or vegetable parchments, cellophane, collodion, or some of the synthetic resins.

In the process described in connection with Figure 1 it is not essential that the solvents in the two solutions be the same although they must be miscible. In the process described in connection with Figure 2 it is essential that the solvents be the same as the solvents mix together due to the continuity of the process. However, difierent The metal-ferrocyanide is the r solvents inay be used in a process somewhat similar to the one shown in Figure 2; if these different solvents are separated somewhere, such, for example, as by a differential distillation process in connection with jar 22.

For obtaining crystals of a given crystalline material by the described device the only'latitude in making the solution I2 is the choice of the solvent, and once the solvent is chosen the only latitude in making the second solution I6 is the choice of the solute. The solvent and'the solutes must, however, be such as to produce in the sec ond solution at least a somewhat higher osmotic pressure than in the first solution. The higher the osmotic pressure difference, however, the better, as the process will be faster. It is also desirable that the solute in the second solution be one with large molecules to prevent back diffusion through the membrane and consequent contamination of the first solution. This requirement is satisfied more often by organic than by inorganic substances. There are, however, only relatively few inexpensive solid organic compounds with a. molar solubility high enough for the requirements of the method. A selection of some of them is tabulated below, together with their molar. solubilities in water at 25 degrees centigrade:

Instead of solutions of solid organic substances, liquids with high osmotic pressure and miscible with or soluble in the solvent of the first solution may also be used in place of the second solution. The heavy alcohols, for example, glycerol, usually satisfy the requirements. In some cases inorganic compounds (electrolytes) with high solubility may alsoserve the purpose.

It has been pointed out that it is important to establish a reasonably high difference between the osmotic pressures of the two solutions involved in the described method. In the beginning, however, the osmotic pressures as such are usually not known, but only the solubilities. These are, in high concentrates and in electrolytes, only a rough indication for the osmotic pressure to be expected. In general, it is, therefore, advisable for a given substance to be crystallized, to choose the solute of the second solution so that it has a substantially higher molar solubility than the substance to be crystallized. This .safety margin should be established for three reasons: (1) The substance to be crystallized, usually an electrolyte, will have a higher osmotic pressure than that corresponding to its solubility, depending upon its degree of dissociation. (2) The eflect of supersaturation will add another few percent to its solubility and osmotic pressure above the values for saturation. (3) A high osmotic pressure differential accelerates the process.

As an example, a few solubility data shall be given (at 25 degrees Centigrade) of some inorganic compounds which may have to be crystallized. Sodium chloride has been listed as a representative of a substancewith an almost horizontal solubility curve. It should be noted that in most any such case the .diiference in osmotic pressures can easily be increased by merely increasing the temperature at which the crystals are grown: The organic substances, as listed above, usually rapidly increase their solubility and osmotic pressure with temperature, while the 7" solubility of a substance with essentially horizontal solubility curve hardly changes." 7 r Molar Solo.-

It is also within the scope of my invention to replace the second solution 16 with a gas, for instance air, whose humidity is controlled. The solvent passing through the membrane will thenevaporate on its outside, thus concentrating the solution within the container without the danger of contaminating it by dust.

Th device for practicing the'aforementioned electro-osmotic method is schematically illustrated in Figure 3. The'physical set-up is the same as in the device shown in Figure l, with the exception that two electrodes 40 and 41 made of platinum, carbon or some other inertmaterial have been added. Each of the electrodes is in contact with the liquid in one of the compartments 6 and I. A battery 42 is connected through an adjustable resistor 43 to the electrodes 40, 41 so that a direct current potential difference is established between the two compartments 8, 1. Such an arrangement increases the osmotic flow of solvent from one compartment into the other beyond the rate at which it occurs due to the osmotic pressure difference between the two compartments without the potential difference. The polarity and magnitude of the potential difference depends upon the nature and concentration of the two solutions.

While I have described my invention with a certain degree of particularity, it is to be understood that this description has been presented only by way of example, and that numerous changes in the steps of the method and numerous changes in the apparatus may be made without departing from the spirit and scope of the invention as hereinafter claimed.

1 claim as my invention:

1. The method of growing a large clear crystal of a crystalline material whose solubility does not substantially increase with temperature which comprises the steps of: planting a seed piece of the crystal to be grown in a first substantially saturated solution of the crystalline material, transferring solvent out of said first substantially saturated solution by osmotic action through a semi-permeable membrane into a second solution having higher osmotic pressure and different solute than said first solution to causesaid' first solution to become supersaturated and'to cause the solute of said first solution to deposit as crystalline material on said seed piece, and adding afresh supply of substantially saturated first sohition'to said supersaturated solution while the solute of said first solution is being deposited on said seed piece.

2. The method of growing a large clear crystal of a crystalline material whose solubilit'ydoes not substantially increase with temperature which comprises the steps of: planting a seed piece of the crystal to be grown in a first sub stantially saturated solution of the crystalline material, transferring solvent out of said first substantially saturated solution by osmotic action through a semi-permeablemembrane into a second solution having higher osmotic pressure and having different solute than said first solution to cause said first solution to become supersaturated and to cause the solute of the first solution to deposit as crystalline material on said seed piece,

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,777,057 Urbain Sept. 30, 1930 2,408,625 Graham et a1. Oct. 1, 1946 2,42%,273 Haas July 22, 1947 FOREIGN PATENTS Number Country Date 20,380 Great Britain July 7, 1910 77,991 Germany Nov. 12, 189d OTHER REFERENCES Glasstone, Textbook of Physical Chemistry,

194.0, 6th printing, page 642, Figure 144. 

